set source ip for logstash output

We've got machines with multiple interfaces and source based routing configured so we can bind applications to specific IPs and they will route over the correct interface. I don't see any way to configure the source IP for the logstash output. It would be nice to be able to bind to either an IP (or an interface) with something like this:

```
output.logstash:
hosts: ["logstash-host:5044"]
source_ip: 192.168.0.100
```

The issue names the Logstash output and proposes a source_ip setting, but it does not identify files, entry points, or tests. Start by locating the Go implementation and configuration handling for that output, then find related output tests; done means a configured source IP or interface controls the outbound connection and the behavior is covered by tests.

Written by the indexing model from the issue text.